Process for the production of oil of low mobility



R. L. REED ETAL 2 Sheets-Sheet l Dec. 1, 1964 PROCESS Foa THE PRODUCTION 0F OIL. OF LOWv MOBILITY Filed May 21, 1962 Dec. 1, 1964 R. REED ETAL 3,159,216

n PROCESS FOR THE PRODUCTION OF' OIL OF' LOW MOBILITY Filed May 2l, 1962 2 Sheets-Sheet 2 United States Patent O 3,1S,26 v PRCESS FR THE PRODUQTN @E @EL GF LW MOBHLHY Ronald L Reed, Allison Park, Pani L. 'few/linger, @latrrnont, and Lawrence A. Wilson, r., Unity, Pa., assignors'to Gulf Research @c Development Company, Pittsburgh, Pa., a corporation of Delaware Filed May 2l, 1962, Ser. No. @6,2% 6 Claims. (ill. loo-ll) This invention relates to the production of oil from underground oil-bearing formations and more particularly to the production of oil having a low mobility at the conditions normally existing in an oil-bearing formation.

Vast quantities of oil which are not susceptible to recovery by conventional production techniques exist in known deposits frequently referred to as tar sands. -Economical production of oil from such formations by the conventional primary or secondary techniques is prevented by the high viscosity or high pour point of the coil at the reservoir conditions which makes the oil substantially immobile.

. A secondary recovery technique that has been suggested for the recovery of highly viscous oils is the insitu combustion process in which lan Yoxygen-containing gas is vinjected into the formation and oil in the formation is ignited .to cause combustion of oil in place in the subsurface formation. A principal purpose of the irl-situ combustion process is to raise -the temperaturel of the oilbearing formation high enough for the oil to become mobile and ioW to a well. Production is further increased by the products of combustion displacing some of the oil from .the formation.

One ofthe in-situ combustion processes that has been suggested is the forward burning process in vwhich oil in the subsurface formation is ignited Aadjacent an injection well used to supply an oxygen-containing'gas, kusually j air, for the combustion. -In the forward `burning process the oxygen-.containing gas-andthe combustion front move in v.the same direction. -Forward burning `is not an effective means for the recovery ofoils of low mobility Vfrom subsurface formations because a bank of iluid oil, gas, and water immediately ahead of the combustion front is prevented from 4iiowing to the production well by the highly viscous cold oil surrounding the yproduction well. The blocking of the flow of oil to the production well prohibits .injection of oxygen-containing gas vinto the oilbearing formation at a rate high enough to maintain combustion.

Another in-situ combustion method that has been suggested for .use in recovery of heavy oils is reverse combustion. 'In reverse combustion processes, an oxygencontaining gas is displaced vdown an injection well and into the oil-bearing formation. Oil in the formation adjacent the production well is heated to a temperature at which the oil will ignite whencontacted with air traveling toward the production well from the injection well. Continued displacement of the oxygen-containing gas into the oil-bearing formation causes the combustion front to move toward the injection Well, countercurrent Vto the ow of gas. 1n the reverse combustion process, the underground formation downstream from the combustion front is hot; hence the oil flows readily to the production well. However, if the underground formation has a very low permeability, it may not ,be .possible to Vdisplace the oxygen-containing gas through the formation tothe combustion front at a high enough rate to maintain combustion. Moreover, low temperature oxidation between the injection well and .the vcombustion front consumes much of the oxygen and may cause little loxygen to reach the combustion front. Reverse combustion has rice the further disadvantage of causing a substantial portion of the oil present in the formation to remain unrecoverable in the formation as coke.

This invention resides in a novel in-situ combustion process for the recovery of oils of low mobility from an underground oil-bearing formation penetrated by an injection well and a production Well spaced from vvthe injection well. lInitially, a narrow hot streak of substantial permeability extending through the oil-bearing formation vfrom the injection well to the production well is created. The hot streak includes a fracture extending substantially horizontally from the production well propped open to form a channel of high flow-capacity and an unpropped fracture extending from the injection well toward the production well. Both fractures are heated prior to ignition of the oil in the formation to ypro-vide the hot streak connecting the wells. An oxygen-containing `gas is injected into the fracture at the injection Well and oil in the formation is ignited adjacent the injection well. Continued injection of the oxygen-containing gas causes burning o-f oil in the formation and `flow of heated oil through the hot streak tothe production `well and heating 'of the subsurface formation .whereby :the fracture communicating with the injection well closes behind the combustion front.

ln the drawings:

FIGURE l is a diagrammatic view in vertical ysection of an injection well and a production well kas equipped for heating the fracture extending from the injection well and with tubing for delivery of oil yfrom the production well; Y

FIGURE 2 is a diagrammatic view, partially in vertical section, illustrating the progress of the heated zone through the oil-bearing formation during the recovery of oil in the process of this invention; and

FIGURE 3 is a plan view of a five-spot arrangement of wells with the limits of idealized fract-ures indicated by broken lines. v

Referring to FIGURE l of the drawings, an underground oil-bearing formation 16 is shown penetrated by an injection well l2 and a production Well 14, 4The oil in oil-bearing formation l@ should have a low mobility at reservoir conditions and must have a substantial change in Viscosity with a moderate change in temperature. The term low mobility refers to mobilities less than-one when the mobility is the ratio of the permeability of the formation in millidarcys to the viscosity of the oil at reservoir temperature in centipoises. Oils capable of the substantial change in viscosity essential to the process of this invention with a moderate change in temperature have a viscosity exceeding 50 centipoises at reservoir conditions. The low mobility of the oil can be caused by a high pour point of the oil or by the presence of highly viscous bituminous material in the oil. Oilbearing formations suitable for treatment in accordance with this invention are frequently referred lto as tar sands, rock asphalt, bituminous sands, and bituminous rock. For convenience, formation l0 is hereinafter vreferred to as a tar sand; however, it is `to be understood that the term is used in a generic sense to include any of the formations listed in the preceding sentence containing an oil of low mobility as defined above.

After the injection well l2 is drilled through the tar sand lil into base rock le, casing l is run into the well and cemented in place by any conventional cementing technique. ln FIGURE 1, `well l?, is illustrated with a burner 2d supported in the borehole a short distance above the lower boundary of the tar sand at the vlower end of a fuel supply line 2.2 which extends down the well within tubing 2d. The fuel line 22 is connected at its upper end to a fuel supply line 26. An air supply line 2r, is connected into the tubing 24 at lthe well head.

. isdescribed in Patent No. 2,668,592@v ljiros. ,l is also provided with a side. outlet 3d at the upper end of ythe casing forthe yintroduction of a suitable diluent into Vj' 'Y .,V

"Burner 20, which is illustrated diagrainmaticaiiy, ispro- Y vided with suitable openings for adrnixture of air delivered through the annulus between fuel line EZ and tub-ing 24 with the fuel delivered through line 22. A suitable burner the casing to provide temperature control in the manner hereinafter described. vProduction well i4 is spaced'fromrthe 1li-(n), i4(b), 14(0), and llid) are spacedV around an injectionfweli 12.V The arrangement of wells illustrated may be part of a series of repeated five-spot patterns. Of'course, arrangements ,of injection andv production'wells other Ythan the4 five-spot arrangement illustrated in FIG- `URE 3 can be used.V The particular arrangement and spacing of the wells used wili depend upon the characteristics 'of the tar sand it? andthe plansfor development of the oil iield. y Y

Production well i4v has cemented in place in accordance withconventional techniques `for setting casing.

If desirable, a pump can be installed in After casing 32 isset in production well it, a sub-.

I ,stantially horizontal fracture d2 is made to Vextend from each of the production V.wells td into vtar sand l@ near the bottom boundarythereof. vTheffractureis preferablyY created by cutting an opening in thecasin'g 52 at the deinjection well. 12 -in-accordancevwith the plan for developing the voilV `iield. AV suitable arrangement is a iive-spotpattern such `'as is illustrated in FIGURE 3 in which production wells casing 32 Vextending through" the tarsand i to the base rock fio. Casing 32 may be siredflocation of the fracture and cutting'a horizontal notchfin the tar sand i0 to fix the location and plane of the fracture. A fracturing fluid such as an oleaginous or aqueous liquid, which may or may not contain a thickening agent, is then pumped down thewell and the pres-. Vsure on the fracturing huid increased to overcome the pressurel of the voverburden and thev tensile strength of the` tarsanditl. After the fracture is created, vasindicated by a dropinvthe pressure-onthe fracturing iuid,

extend ita desired distance from the Well is. A liquid coutaining'a heat-resistant propping agent V=is.fthen dis-` placed 'into the fracture to provide, a'fracture having a fluid ow capacity several times the capacity of the tar sand 19 prior to racturing. A' preferred propping agent is'4 to ,-8 mesh sand. It is preferredy that the fracture d2 extend at least about 20 to 30 percent ofthe distance to lthe injectionrwell i2. A suitable process for creating the fracture .isrdescribed in PatentNO.V 2,699,212 of `hlewton B. Dismukes. The process` described in that'patentfor vthe creation of a vertical fracture can readily be modified j of combustion fromv the burner are passed outwardly through the fractures. The heating of the fractures is continued until the portion of the tar sand withinapproximateiy three feetof the fracture is heated to'a ternperature in the range of 400 to 700 F. fora distance of 20 to 30 percent of the distance from Athe productionk well to the injection well.

It is important to restrict the heating Vto a narrow zone adjacent the surfaces of the fracture; otherwise the amount of heat required to lraise the tar sand adjacent the fracture tothe desired temperature is excessive, along tirneisV required' toV supply the large amount of heat required, and the heat lost to the surrounding formation is large. Heating lof the formation above the fracture renders oil present'in that vportion of the tar sand fluidand results in migrationV of hot gases injected into fracture 42 upwardiy into thev formation.V Such migration encourages the creation of a fracture at an upper. levelin thetar sand. Heating of the tar sauditi above the acture can be prevented byicirculation of a' cooling fluid such f as inert gases, air, or water down the annulus surrounding the burner. The cooling` uid further providesicontrol of the maximum temperature ofthe combustion products entering the fracture 42 to a temperature in therange of 400 to .1200D F., and preferabiy from400 to700 F Y The exact gas rate andV injection gas temperatures Aused to heat the tar sand Vadjacent fracture 42 will beV different for each reservoir and well spacing, but *it `is important that the tar sand within one to three feetof thefracture be heated tof the desired ternperaturev'i'nV a periodi of one to three weeks The'minimum rate of injection yof hotgas'es vata temperaturelrwithin the limits of 400"Y Rito 1200 F. toaccomplish the desired heati ing can -be calculated from the formula Q=v155L2`when Q isV thegas injectionvrate in standard cubic feet per day i and L is the distance-in feet between the injeotionfand production Wells. In a :five-spot pattern with a well spac-` ing Iof 250 feetbetween produetion'wel'ls', the injection of a gas at a. temperature of 675 F. ,at a, rate of 10,000,` 000 std. cu. ft. per day into the fracture will heat the' `tar sand for a distance of .three feet on either side of' v ythe fracture to a ytemperature of400 F. for a distance of about 6,0 feetffrom the production Wellin approxi- Y mately '14 days.` '.V'If thek rate ofow of the gases is rei additional fracturing fluid is pumped into thefracture to f to create the vdesired horizontal fracture `i2 by initially f cutting a horizontal notchin the borehole'wall.

yWhen a tive-spot arrangement of the'typ'e illustratedy 7 inv FIGURE 3 is employed, a fracture 42 is created at each of the production wells it-(a), Mtb), 1Mo), .and 1401). The extent of thefractures extending from the production 'wells is indicated diagrammatically in FG- URE 3 bythe broken lines 42(4), 42(17), 432(c), and '42(d),

' After the creation of the fractures d2; extending fromv eachV of the production Wells, the surfaces of the fractures and a narrow zone of the tar sandwithin about three feet above andy below'the fractures are rapidly heated by passing a hot gas through the fracture. For this purpose, a burner, which may be yof the type illustrated in FIG- UREl in the injection Well 12, is located lin the production well i4 opposite the Vfracture 42,' and the products duced to VV1,0()0|,000 std. cu. ft. per day, approximately one yearis' required before the desired heat distribution alongthe fractureis obtained.

' f A preferred method rof, obtaining the desired heating of theftar sandl adjacent the fractures communicating'. with the production Wellin .a five-spot pattern is to cir-V cula-te the hot combustion products from Voneproduction i' well to another. V.For example, referring toFIGURE 3 of the drawings, a burner can be installed in each'of. v wells 14 (a)-and (ldd).

Vdisplaced at the rates and temperatures mentioned above ,into the `fractures communicatingwith'those wells and Hot combustion "products are discharged through wellslHc) and 14(1)). "After the fractures adjacent the Vweils. 1401) and 14.(61) have acquired the desired temperature.distribution, burners can-1 be installed in Wells 14(0).and 14(b)' and the Vhot cornbustion products from those burners vented' through wells I jl 1401)' and 14M). Informations'of low effective penne-iV Y ability to gas,"Y it may not be possible to cause the Yhigh Vgas volumes required for the'heating stepv toY iiowffrorn j one production` Well Vto 4another at Vnormal injection pressures.

pneumatically extended from one Aproduction well to another. VAfter the heating has beenv completed and the injection pressure released, the pneumatically extended 1 'fracture will close and leave only` theY propped VYportion of the fracture open. After heating of the fracture 42, `has been completed, the burner is pulled from the produc-V tion wells 14, andthe production tubing 34 is run through In such instances, the injection pressure on the hotl gases can be increased. until the propped fracture-isV the fracture. Casing 32 may be perforated at 43 through the tar sand 1t) before lowering the tubing.

A substantially horizontal fracture 44 iscreated from Athe injection well 12, preferably by notching the formation at the desired depth in the injection well 12 and then displacing a .fracturing fluid free of propping agent down the injection well under a pressure adequate to create a fracture. Fracture 44 .is extended at least about 80 percent of the distance tto the production well 14. The fracture 44 should be located at adepth near the lower boundary of tar sand such that fracture 44 will be close .to fracture 42 whereby, during the subsequent heating step, a narrow .hot zone extending from .the injection well to the production well is created in the tar sand 10. The fracture 44 may intersect the fracture 42, but such intersection is not .essential to the process of this invention. With .this arrangement, .the portion of the tar sand between the fractures 42 and 44 is heated, and because of the large overlapping area of the fractures 44 and 42, resistance to iiow into the fracture 42 is low.

The tar sand 10 within about one foot `and preferably within about three feet of the fracture 44 is heated to a temperature not less than about 200 F., depending .on the characteristics of the oil, for a .radial ydistance yof about 70 to s80 percent of the 4distance from .the injection wel-l to the rproduction well.infa period of one .to three weeks, and preferably in a period of one to two weeks. During the period of heating the fracture .and a zone above and below it, the injection pressure is maintained high enough to support the overburden and thereby hold the fracture open. An indication of the extent of the heating that has been 4accomplished will be obtained from the temperature of gases that break through into the production well. It is imperative .that the heating of the tar sand adjacent fracture 44 be obtained by a very high rate of flow of hot gases into fracture 44 to obtain a substantially uniform flow pattern from the injection well to cause flow of oil through the formation to each of the production wells. .It has been found that a low rate of injection of hot gases into the fracture will result in initial migration of hot gases preferentially to one of the production wells because of nonuniformity of .the fracture. The heating of the fracture faces and tar sand by those gases further reduces the resistance to flow through the tar sand and results in channeling of the hot gases to one of the production wells, preferentially to the outer wells. High rates .of

injection greatly diminish the tendency to channel to one well.

The Vheating of the tar sand adjacent fracture 44 can be accomplished by burning-a fuel such as lease gas, propane, gas oil, etc. supplied through supply line .26 to the burner adjacent the intersection of the fracture 44 with well 12. The temperature o f the gases displaced into the fracture can be controlled by the inert gas, water, or air delivered into the well through line 30. I-f inert gases or water are introduced through line for the desired temperature control, the injection gas 'temperature should 'be 400 to 1200 F. If .excess air is used to control the temperature of the `hot gases, the maximum gas temperature should be no higher than 700 F., and preferably labout 400 F., to prevent migration of air into the upper portion of the tar sand adjacent the injection well, with the consequent liberation of heat by burning oil present in the tar sand. The burning of oil in upper regions of the tar sand at this stage is to be avoided because it is imperative to restrict the heating of the tar sand to a thin zone adjacent the fracture in order to obtain the breakthrough of heat extending from the injection well to the production well.

Extremely high rates of viiow of hot gases into the fracture 44 are essential in this step. The minimum rate of displacing hot gas into the fracture at the injection `well can be calculated from the formula Q=1400L2 when Q is the volume of gas in standard cubic feet per vday andkL is the distancebetween injection well and production well in feet. For example, if Vthe well spacing in a five-spot pattern is 250 feet and the Atemperature of the lgases injected into the fracture is 675 F., -a rate of flow of the gases of approximately 100,000,000'stdycu. ft. per day is required to obtain the desired Vheat distribution in a l-4-day period. The heating of 'fracture 42 and frac- .ture 44 can be accomplished simultaneously. `Such aprocedure has the advantage of reducing lheat losses from the -tar sand adjacent yfracture 42 while fracture 44 is heated, and it is preferred ifk adequate'compressor capacityvis available.

'Ifthe -upper #portion of vformation 10 adjacent well ki2 ris heatedexcessively during the period of heating'the 4faces sof the fracture 44,-thefracture tends to migrate upwardly during `the heating. Air subsequently injected kmay then flow across the -top of the vformation land bypass the oil event-burner 20 is Vnot required. A pyrophoric liquid "such as triethylborane 4is displaced outwardly into the l'fracture with an inertfluidsuch as diesel oil.

A fuel-air mixture is theninjected vinto the fracture. Contact of the air with the pyrophoric liquid will `cause 4ignition 'of the pyrophoric material Vfollowed Vby ignition of the fuel. VAdjust-ment of the composition of the Vfuel-air mixture and the rate of injection `of the mixture into the'fracture prevents burning back Vto the well. For example, if propane is `used as-the fuel, a fuelair ratio of about 4-12 -percent of -the stoichiometric ratio will prevent burning back to the well. Injection of the fuel-air mixture is continuedto heat lthe formation adjacent fracture 44 to the des'ired'temperature.

When `the tar -sand within about lthree `feet of the fracture 44 has been heated to a ltemperature of 200 to l400" F. for a distance of about 70 to 80 percent of the distance vbetween the injection well Iand lproduction "well, fuel injection into vthe Linjection well 12 is stopped, but

the air -injection -is continued. At this time, the rate of the injection of the air may be reduced. It is only essential `that the rate be high enough to maintain combustion and overcome losses of heat from the heated zone adjacent rthe fractures 44and 42.A Because of the difference in density -of the air and the oil in Athetar sand, air injected into the `fracture migrates upwardly into the tar sand 10 and 'burns a portion of the oil in the tar sand at some distance from the fracture 42. The heat released by combustion of a portion of the 'tar sand raises the ternperature of the `tar sand and thereby increases ,the mobility of theoil remaining lin the-heated portion. The oil is gas driven through the tar sandintofracture 44 and is driven outwardly in yfracture 44 'to the portion of that fracture overlapping fracture 42. The combination of the large overlapping area of the fractures andthe preheating of the intervening portion ofthe pay Yzone allows the oil to move into fracture 42 and through that fracture to the production .well 14.

Because the oil Iowing into fracture 42 is hot, it does not cool the fracture and will supply whatever heat necessary `to :replace the Igradual loss of heat from the formation adjacent fracture 42. However7 unless fracture 42 is preheated, the heated oils yfiowing into it are rapidiy cooled lto .a temperature at which they become highly viscous and quickly plug the fracture. Infield tests, fractures opened all of the way 'between an injection well and certain production wells have become kplugged by cold oildur-ing the combustion step; however, once production of hot loil was obtained, no further plugging of the fracture was experienced.

Upon continued injection of air into the fracture 42, the air migrates upwardly vinto the formation because of the -density difference between the air and oil. The air 9 lburns oil draining through ,which gradually raises the boundary of the heated zone .l until the tar sand is heatcdsubstantially all Vof the way to adjacent the injection well. ,jacent the injection well closes, and thereby further in-` `of about 2,000 millidarcys.

created in each ofthe wells four feet above the` lowerY the tar Vsand andy releases Vheat its upper boundary. For purposes of illustration, the

' boundary of the heated zone at an intermediate period in Y the productionl of oil from the tar `sand is illustrated in FGURE 2 by line do. The Aheating of the tar sand withY v3,11v 559,721 e Y Y Y a 1500 centipoises.l inately20'to 3,0 centipoises at 200 F., and the Vviscosity is further reduced bythe dispersion of C02 init. The

' ratio of air injected to produce oil was 28,000std. cu.

the consequent removal of immobile oil from it, the de- Y hydration of clays that may be present, andthe spalling of kthe roel; increases the permeability of the tar sand As Va result, [the fracture adcreases the tendency of the injected gases to llow upwardly Vinto the tar sand to cause a moreuniforrn ow of injected gases through .the full thickness'of the tar sand. The

low permeability of the portion of the tar sand 10 to the f right of line 46 in FIGURE 2 results in a substantial difference in pressure in Vthaty portion of the tar sand and v the Outer portion of fracture dtgrfhence, the outer portion of fracture 44 remains open andV provides achannel for the flow of'oil through the tar sand beyond the, combustionfront. f f Y 1. y y In a specific embodiment of this invention, yfour production wells aredrilled at the corners of'a ve-spot pat- .tern with spacing of 250 feet between the wells into a tarsand at a depth of 200 feet, having va thickness of ,24 feet, a porosityof 22. percent,` and an air permeability Y Horizontal fractures .are

boundary` of the tar sand and are propped open with 4to 8 mesh sand. Burners are installed in diagonally opposite c V.wells adjacent the intersection ofthe fracturerwith Vthe. borehole of the wells and propanek burned; at a rateand with excess air to displace 10,000,000 std. cu. ft. per day ofcombustion products at a temperature of A67,59 F. into th'effracturesg The heating of the fractures is continued for 14 days. Thereafter, therburners are transferred to` ,thel other two production wells and the procedure repeatedk u f -for a second 14-day period.

y. VV.Arrinjection well is drilled through thetar 'sand at the center ofthe tive-spot pattern and a substantially hori 1 f zontal fracture is: created in. the'tarV sand Vabout five' feet ,aboveVv itsf lower boundary.y The fractureis extended from the injection Well by the displacementof an aqueous fracturing liquid free of proppingl agent and containing aboutone Ypound per gallon of a gelling agent'approximately 80 percent of the distance to the production wells."

A burner'is run into the injection well to la position opi posite thevfracture. down the well yand burned adjacent the fracture, and the @hot combustion products Vdisplaced into the fracture. e

Water is displaced down the well at arate to cool the Air 'and propane gas are displaced,y

products of vcombustion to a'temperature,ofapproxify mately y900' F. Theinjection of the propane, air, and j; water is controlled to produce a flow Vrate of 100,000,000. std. cu. ft; per day of gas'at a ltemperature lof approximately 900 F. into the fracture. The'heatingoffthe' fracture yextending from the injection well is continued fr'lfor` 14days. v. j y Y.

'After heating theffracture communicatingwith Vthe injectiony well for fourteenz days, the fuel ysupply and cooling Water are stopped: andair linjection isV continued at a reduced rate of approximately 500,000 std. cu. ft. perV day'. Oil `in the formationis burnedwhich heats other oil that travels radially outward from the injection well to the fractures communicating with the production well and through those fractures to thefproduction well.

The rate'of injection of air. into the injection wellris controlled to maintain less than 5: percent'oxyge'n in the gases discharged from the production well. If'the oxygen of the air is decreased.A v A v .Inja test of the foregoing process, it was found that u concentration rises above 5 percent, thef rate of injection ,7. "llA method of Vrecovering an ,oil

' from an oil-bearing formationcontaining'saidcil penej trated by a production well and aninjection well corn-V ft. Vper barrel. The produced oil was suitable for use Va .No.6 fuel oil without further processing.` y

We claim: y v Y of low mobility prising creating a substantially horizontal fractureextending ,from :the production wellinto the oil-bearing formationV nearthe lower bo'undary'thereof, depositing a heat resistant propping agentk in the fracture, displacing via the? production vwell a hot gas `at aVVV temperature of 400? F. to `1200 F. into the fracture at a high rate t0 heat the formation within one foot iof the fracturefto a temperature of latleast 200 VF. in a period of'less than f yaboutellt days,'creating a'second fractureV extending fromY the injection well :substantially horizontally into the oil- `uing the vinjection of oxygen-containing gas viathejinjecl `tionwell into the' second fracture Vto` cause combustion Y v of :oil iny the formation toheat oil to altempfratue atKV l' which it flows throughthe rst 'fracturer'into the production'welL'and lifting oilffrom the production well.''

2; A'method of recovering an oil of low 5"mobility fromV an oil-bearing formation containing said oil, said oilbearing formation'being penetrated by an injectionrw'ell and a production well Vspaced therefrom, comprising creating asubstantially horizontal lirst fracture extending from the production well into the oil-bearing formation near the lower boundary thereof, depositing a heat-reek sistant -propping yagent in the fractureLdisplacing yvia the Y production well-.and into the'fracture a'hot gas at a rate of at least 155 L2 std. cu. ft. per day where Lis the distancev-in'feetbetween theV injection well and production well and aV temperautre of V400" to l200 F. to heat theV ,Y

foil-bearing formation within threevfeet ofthe'fracture to.A f

'y a temperature of at 1east'2'00 F., Vcreatinga second frac-v* ture extending from vthe injection wellsubstantially hori-` zontally into the oil-bearingformation near the lower.

Y boundray thereof.' displacing via the injection well ja: gas

at-a temperature of-,400c' to'1200? F. and a rate of at least 140,0 L2 std. cu. ft.vperday into thesecond fractureto heatfthe oil-bearing formation within threeffeet of the second fractureV to. a temperature Y ofatfflcas't 200 F.,

said second fracture `being free 'of Vpropping agent, dis-m Y continuing the displacement of hotlgas'into theffracture's, Y displacing via theinjection well van oxygen-,containing gas; .K into. the second fracture, igniting oil in the formation ad` jacent the injection -welland .continuing 'the injection of oxygen-containing gas-'viaV the Vrinjection well into the second fracture ,to cause combustion of oil in the Aforma- Vtion, and` thereby'heat' oil in the oil-bearing formation to a A temperature at whichrit flows lto the iirst fracture andI through the iirst fracture to thev production well, and f lifting oil from the production well to the surface.

3. A method of recovering an oil of low mobility from an oil-bearingforrnation containing said oil, said oil-bearan oil having a viscosity of 50,000 centipoises at F.

Y was upgraded to anoilhaving a viscosity at"70 F. of

ingformation being penetrated by an injection well and a production well spaced therefrom, comprising creating ra substantially horizotnal firstr fracture extending from the Vproduction' welly into' the Aoil-bearing formation 'near'. the lower boundary thereof, depositing a heat-resistant propping agent in the fracture, displacing'via the produc-t tion'well and into the fracture a hot gas ata rate of .at

Such oil has a viscosity of'approiih" Y least 155 L2 std. cu. ft. per day Where L is the distance in feet between the injection well and production well and a temperature of 400 to 700 F. to heat the oilbearing formation Within three feet of the fracture to a temperature of at least 200 F., creating a second fracture extending from the injection well substantially horizontally into the oil-bearing formation near the lower boundary thereof, displacing via the injection Well a gas at a temperature of 400 to 700 F. and a rate of at least 1400 L2 std. cu. ft. per day into the second fracture to heat the oil-bearing formation within three feet of the second fracture to a temperature of at least 200 F., said second fracture being free of propping agent, discontinuing the displacement of hot gas into the fractures, displacing Via the injection well an oxygen-containing gas into the second fracture, igniting oil in the formation adjacent the injection well and continuing the injection of oxygen-containing gas via the injection well into the second fracture to cause combustion of oil in the formation and thereby heat oil in the oil-bearing formation to a temperature at which it flows to the first fracture and through the rst fracture to the production well, and lifting oil from the production well to the surface.

4. A method of recovering an oil of low mobility from an oil-bearing formation containing said oil penetrated by a production well and an injection well comprising creating a substantially horizontal fracture extending from the production Well into the oil-bearing formation near the lower boundary thereof, depositing a heat-resistant propping agent in the fracture, displacing via the production well a hot gas at a temperature of 400 F. to 1200 F. into the fracture at a high rate to heat the formt tion within one foot of the fracture to a temperature of at least 200 F., discontinuing the displacement of hot gas via the production Well into the fracture, creating a second fracture extending from the injection well substantially horizontally into the oil-bearing formation near the lower boundary thereof, displacing via the injection well a mixture of a fuel and oxygencontaining gas into the second fracture, igniting the mixture of fuel and oxygen-containing gas and burning the fuel in the second fracture to heat the oil-burning formation within three feet of the second fracture to a temperature at least 200 F., discontinuing the displacement of fuel into the second fracture, continuing the injection of oxygen-containing gas Via the injection well into the second fracture to cause combustion of oil in the formation to heat oil to a temperature at which it ows through the rst fracture into the production well, and lifting oil from the production well.

5. A` method of recovering an oil of low mobility from an underground oil-bearing formation containing said oil, said oil-bearing formation being penetrated -by a production well and an injection Well spaced therefrom, comprising creating a substantially horizontal rst fracture extending from the production well into the oil-bearing formation near the lower boundary thereof for a distance of at least about 20 percent of the distance to the injection well, depositing sand in the rst fracture to prop the fracture open, burning a fuel in the borehole of the production well adjacent the first fracture to produce hot combustion gases, diluting the hot combustion gases with a non-combustible gas to lform a mixture of gases at a temperature in the range of 400 to 1200 F., displacing said mixture of gases into the irst fracture at a rate of at least 155 L2 std. cu. ft. per day Where L is the distance in feet between the injection and production Wells for a period of about 14 days to heat the oil-bearing formation Within three feet of the fracture to a temperature of at least 200 F., creating a second fracture extending frorn the injection Well substantially horizontally into the oil-bearing formation near the lower boundary thereof for a distance of at least about percent of the distance from the injection well to the production well, injecting at a rate of at least 1400 L2 std. cu. ft. per day via the injection well and into the second fracture hot gases at a temperature of 400 to 1200o F. and a pressure adequate to maintain the second fracture open to heat the formation Within three feet of the fracture to a temperature of at least 200 F. in a period less than about three weeks, discontinuing the injection of the hot gases into the fractures, displacing air Via the injection well into the second fracture to ignite oil in the formation adjacent the injection well, ,continuing the injection of air into the second fracture to move a combustion front through the oil-bearing formation from the vicinity of the injection well toward the production well whereby the second fracture closes behind the combustion front, and lifting oil through the production well.

6. A method as set forth in claim 5 in which the injection and production wells are located in a live-spot pattern with the production wells at the corners of the ve-spot pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,818,118 Dixon Dec. 31, 1957 2,946,382 Tek et a1 July 26, 1960 2,962,095 Morse Nov. 29, 1960 OTHER REFERENCES McNiel, I. S., Ir., and Moss, I. T.: Oil Recovery by In- Situ Combustion, The' Petroleum Engineer, July 1958, page B29 (pages B-29 to 32, 36, 41 and 42 relied on).

UNITED STATES PATENT QFFICE CERTIFICATE OF CCRRECTION Patent Ne. 3,159,216 December 1, 1964 Ronald L. Reed et al.v

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Lettere Patent should read as corrected below Column l, line 20, for "coil" read oil column 9, line 42, for "oilbu`rning" read oil-bearing".

Signed and sealed this 20th day of April 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Amseting Officer Commissioner of Patents` 

4. A METHOD OF RECOVERING AN OIL OF LOW MOBILITY FROM AN OIL-BEARING FORMATION CONTAINING SAID OIL PENETRATED BY A PRODUCTION WELL AND AN INJECTION WELL COMPRISING CREATING A SUBSTANTIALLY HORIZONTAL FRACTURE EXTENDING FROM THE PRODUCTION WELL INTO THE OIL-BEARING FORMATION NEAR THE LOWER BOUNDARY THEREOF, DEPOSITING A HEAT-RESISTANT PROPPING AGENT IN THE FRACTURE, DISPLACING VIA THE PRODUCTION WELL A HOT GAS AT A TEMPERATURE OF 400*F. TO 1200*F. INTO THE FRACTURE AT A HIGH RATE TO HEAT THE FORMATION WITHIN ONE FOOT OF THE FRACTURE TO A TEMPERATURE OF AT LEAST 200*F., DISCONTINUING THE DISPLACEMENT OF HOT GAS VIA THE PRODUCTION WELL INTO THE FRACTURE, CREATING A SECOND FRACTURE EXTENDING FROM THE INJECTION WELL SUBSTANTIALLY HORIZONTALLY INTO THE OIL-BEARING FORMATION NEAR THE LOWER BOUNDARY THEREOF, DISPLACING VIA THE INJECTION WELL A MIXTURE OF A FUEL AND OXYGENCONTAINING GAS INTO THE SECOND FRACTURE, IGNITING THE MIXTURE OF FUEL AND OXYGEN-CONTAINNG GAS AND BURNING THE FUEL IN THE SECOND FRACTURE TO HEAT THE OIL-BURNING FORMATION WITHIN THREE FEET OF THE SECOND FRACTURE TO A TEMPERATURE AT LEAST 200*F., DISCONTINUING THE DISPLACEMENT OF FUEL INTO THE SECOND FRACTURE, CONTINUING THE INJECTION OF OXYGEN-CONTAINING GAS VIA THE INJECTION WELL INTO THE SECOND FRACTURE TO CAUSE COMBUSTION OF OIL IN THE FORMATION TO HEAT OIL TO A TEMPERATURE AT WHICH IT FLOWS THROUGH THE FIRST FRACTURE INTO THE PRODUCTION WELL, AND LIFTING OIL FROM THE PRODUCTION WELL. 